1. Field of the Invention
The invention is related generally to nuclear magnetic resonance (NMR) technology, and more particularly, to methods utilizing NMR to non-invasively inspect unopened containers to screen for liquid explosives and explosive precursors.
2. Related Art
During the past decades, there has been an increased threat of terrorist attacks involving explosives, including Improvised Explosive Devices (IEDs). Because of this increased threat, security concerns are now greater and extend to airports, seaports, rail stations, prisons, embassies, and many other secured and unsecured facilities.
One of the oldest known explosives is gunpowder, a solid explosive made up of potassium nitrate, charcoal, and sulfur. However, liquid explosives can also be manufactured from common chemicals that may be used for legitimate purposes and thus are legally obtainable and readily available. Improvised explosives can be mixtures of an oxidizer, which supplies oxygen to a chemical reaction, and a fuel, which supplies the element that reacts explosively with the oxygen.
Even though explosives in liquid form are highly sensitive to shocks, they may be transported in sealed containers by preparing solutions that keep the compound more stable or by transporting the components needed to form an explosive in separate containers. In general, liquid explosives are chemically easier to combine and simpler to ignite than solid explosives.
Several nitrogen-containing and peroxide-based compounds are used to create homemade explosives, and generally, liquid explosives can be classified as containing either peroxide compounds or the element nitrogen. Hydrogen peroxide is of particular interest for security checkpoints as it may be used as an explosive precursor. Contact between hydrogen peroxide and combustible materials such as wood, paper, or oil, causes spontaneous ignition or combustion. When mixed with materials such as sugars, alcohols, or acetone, the result is a powerful explosive. Hydrogen peroxide chemically resembles water, so it is a good candidate for a smuggled explosive precursor.
Vapor and trace detectors may be used to screen containers for explosives, but these require direct contact with the compounds inside the containers. In addition, emulsion and slurry-based compounds such as ammonium nitrate have low vapor pressure, making them more difficult to detect with vapor and trace detectors. Optical detection technologies such as Raman and infrared spectroscopy have been demonstrated to be effective for detecting some hazardous liquids, but require illumination of the sample and thus are limited by the opacity of the container's walls.
Nuclear magnetic resonance (NMR) has been demonstrated to be an effective method of discriminating between the expected parameters of a known liquid and an altered liquid in an unopened container, that is, ensuring that the liquid specified on the bottle label matches the bottle content. This method, however, is based on content verification, which requires an extensive knowledge base of the NMR responses for all compounds of interest. Hence, the implementation of such a method is not practical as the knowledge database is extremely extensive and constantly changing.
Accordingly, a need exists for a method of detecting those elements always found in liquid explosives quickly, efficiently, and inexpensively, with a high degree of accuracy and a low incidence of false alarms.